The present invention relates generally to the control of unwanted seepage in below-ground environments, and more particularly to a novel method and apparatus for effecting high pressure isolation of mine passageways and the like from liquid seepage.
Underground liquid seepage in mines and other below-ground chambers, such as underground storage facilities, has been a continual problem. Numerous attempts have been made to seal off passageways so as to prevent seepage of unwanted liquids. The problem is most acute where the host rock is an incompetent or soluble rock such as that found in saltrock or near potash deposits.
One known method employed to prevent such seepage involves pouring aggregates of fly ash and cement through a suitable entry to seal the passageway. This technique is only partially effective, however, since a certain amount of high pressure unsaturated brines and other unwanted liquid mobile substances migrate through the microfractures of the surrounding host rock and seepage continues.
A more recent technique for isolating unwanted liquids underground, and which finds particular application in underground rock cavities and reservoirs for storing nuclear residues, comprises surrounding a container for the residues with a viscous liquid such as bitumen or Bentonite having a density somewhat above that of water so that the viscous liquid effects a sealing action in cracks in the container and/or the surrounding rock formation. The container is spaced from the surrounding wall of the rock cavity by means of resilient or flexible supports. See, for example, U.S. Pat. No. 3,925,992. While the technique disclosed in the aforementioned patent is somewhat successful in dealing with underground seepage of pressurized liquids, it has not eliminated the problem.
The problem of underground seepage is particularly acute in deep potash mines, such as located in Canada, where flooding has occurred in salt rocks consisting of more than one mineral such as sylvite, carnallite, and halite. Even at very low pressures, unsaturated brine will frequently bypass an engineering barrier through cracks and fractures in the wall of the mine opening adjacent the barrier. In addition to bedding planes and other lithological discontinuities, there are fracture zones in the area subjected to stress concentrations during and after the excavation of an opening.
Prior to constructing a barrier, all fractured and otherwise potentially discontinuous material must be removed. However, even a complete removal of all such material will not prevent infiltration of brine into the stress relieved zones around the opening. Scanning electronic microscope photographs of potash specimens have shown that stress relieved potash rock is full of microcracks resulting from the difference in strains of the sylvite and halite crystals. Therefore, it has now become known that a successful design of an isolation structure should not only contain an impermeable seal at the interface of the rock and the dam or barrier, but should also prevent migration of the brine through microfracture systems in the vicinity of the isolation structure.